The ornamental design for a gazebo, as shown and described.
FIG. 1 is a side view of a gazebo showing my new design with round posts and a circular support, wherein the view from the opposite side of the gazebo is identical.
FIG. 2 is another side view thereof rotated by 90 degrees relative to FIG. 1, the view from the opposite side being identical.
FIG. 3 is a top perspective view thereof; and,
FIG. 4 is a top plan view thereof.
Broken line areas in the illustrations depict portions of the gazebo that form no part of the claimed design.